Hemodynamic parameters are measurable attributes associated with the circulatory system of a living body, such as, for example, blood flow rate, blood pressure, volume of the vasculature, volume of the cardiac chambers, stroke volume, oxygen consumption, heart sounds, respiration rate, tidal volume, blood gases, pH, and acceleration of the myocardium. There are numerous medical reasons for sensing and tracking changes in hemodynamic parameters, including the proper operation of implantable cardiac stimulation devices.
Implantable cardiac stimulation devices (such as pacemakers, defibrillators, and cardioverters) are designed to monitor and stimulate the heart of a patient who suffers from a cardiac arrhythmia. Using leads connected to the patient's heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to detected cardiac events which are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart's regular rhythm.
Modern implantable devices have a great number of adjustable parameters that can be tailored to a particular patient's therapeutic needs. Any of a number of parameters that define pacing characteristics may be optimized. Adjustable parameters may include, for example, the atrio-ventricular (A-V) delay, the R-R interval, and the pacing mode (e.g. pace and sense in the ventricle, inhibited (VVI), pace and sense in the atrium and the ventricle, both triggered and inhibited (DDD), etc.). As an example, the A-V delay is typically optimized in dual-chamber (atrial and ventricular) pacemakers to time the ventricular contraction such that the contribution of the atrial contraction is maximally exploited. As another example, ventricular synchronization may be optimized in biventricular pacing for heart failure by adjusting the timing at which pacing pulses are delivered to various cardiac sites.
Typically, interchamber pacing intervals (such as A-V delay in dual chamber pacemakers and RV-LV delay in biventricular pacemakers) are set to default nominal values, or else relatively labor-intensive methods are used to measure hemodynamic variables in an effort to optimize some or all of the parameters at the time a cardiac stimulation device is implanted. Examples of measurements that may be carried out in connection with device programming include ultrasound to measure mitral flow and/or ejection fraction and left heart catheterization to measure the rate of change of left ventricular pressure during systole, which is a measure of contractility and mechanical efficiency.
One common technique for setting device parameters involves manually varying the operating parameters of a pacing system while monitoring one or more physiological variables. Typically, the optimum value for a parameter is assumed to be that which produces the maximum or minimum value for the particular physiological variable. This manual method can be time-consuming, during which the underlying physiologic substrate may change and give rise to inaccurate assessment of cardiac performance. Additionally, the manual method is prone to errors occurring during data gathering and transcription.
An automated technique for setting at least one type of device parameter entails systematically scanning through a series of available A-V pulse delays at a fixed heart rate while monitoring a measure of cardiac output, then setting the A-V pulse delay to the value which resulted in the maximum cardiac output. Another technique selects the A-V pulse delay by maximizing the measured value (e.g. by electrical impedance) of a parameter such as stroke volume.
Another method for automatically selecting a cardiac performance parameter entails periodically pacing the heart for a short period of time with stimulating pulses having a modified pacing parameter value, then allowing the heart to return to a baseline value for a relatively long time. The cardiac performance parameter is monitored both during and after the heart is paced to determine if it has improved, degraded, or remained the same. The heart is then paced with a modified pacing parameter value and the process is repeated.
The optimization of pacing parameters is not necessarily critical in patients with relatively normal myocardium, although it may be beneficial to them. These patients have the necessary cardiac reserve to compensate for programming errors. It is patients with depressed cardiac function that are much more sensitive to factors such as pacing rate and A-V delay. Current optimization techniques are time-consuming and labor intensive. Furthermore, they are prone to error because they do not account for variability in the measured hemodynamic signals that often obscures real and significant changes in hemodynamic status and complicates measuring the absolute values of hemodynamic parameters. What is now needed is a method or system for detecting changes in hemodynamic parameters as well as obtaining accurate absolute measurements of such parameters.